1. Field of the Invention
Invention relates to CMOS image sensor design and more particularly to enhancement of image uniformity of CMOS image sensors.
2. Background of the Related Art
There has been an increase of digital image devices using CMOS image sensors. Typically, a CMOS image sensor may be a sensor die that is a piece of silicon and includes an integrated circuit (IC) to function as an image sensor. A conventional sensor die comprises a processing area and a sensing area that may have from several hundred thousands to millions of identical sensor pixels. Hereinafter, for simplicity, the sensor die (or, equivalently CMOS image sensor) refers to its sensing area only.
In general, CMOS image sensor manufacturers test the intensity distribution of the sensor die using a uniform parallel light source. In this case, each pixel collects the uniform amount of optical signal at a fixed angle. Thus, defective pixel can be identified by comparing the output signal from each pixel with those from its neighboring pixels as the defective pixel would generate an unusual output signal.
However, a conventional image device with healthy pixels still can yield non-uniform images, which may be rooted in other sources, such as imaging lens. A conventional image sensor device has an imaging lens that forms an image to the sensor device in which pixels collect optical signals at different ray acceptance angles. Consequently, if the pixel layout is configured to have angular dependency on the ray acceptance angle, the output from pixels can be non-uniform and a function of the ray acceptance angle. Such non-uniformity may yield the non-uniform intensity pattern of the image even though the sensor die comprises healthy pixels.
One solution to improve uniformity might be designing pixel layout to minimize and/or remove angular dependency. However, such design requires lengthy and expensive turn-around processes. Thus, there is a need for image sensors with a system and method to enhance the uniformity by compensating intensity of digital images captured by the image sensors.